This invention relates to an improved laser device and method for obtaining a Raman shifted laser output of high intensity, and with controlled pulse envelope.
In the past, generation of light having Raman shifted frequency has been achieved by providing a Raman medium in a cell external to the laser cavity that generates light at a fundamental frequency. More recently, it has been shown that increased efficiency can be provided by a configuration in which the Raman medium is placed inside the cavity of the fundamental laser, or is itself in a cavity at the Raman frequency that is contained in whole or in part within a cavity at the fundamental frequency. A disadvantage of this later configuration is that a number of optical elements must be provided which are capable of withstanding the intensities of the different frequency light fields. Further, such a system requires the use of dichroic optical elements and dielectric coatings, which are expensive and are susceptible to damage. For a discussion of such a device see U.S. Pat. No. 4,618,783 to Pradere et al. Such devices produce Raman pulses having fixed durations that are longer than a few nanoseconds.
Other Raman type devices are discussed in R. Frey A. Martino and F. Pradere, Optics Letter 8, 437 (1983): R. Frey A. Martino and F. Pradere, IEEE J.Q. Electron QE-20, 786 (1984): F. DeRougemont, D. K. Xian, R. Frey, and F. Pradere, Optics Letters 10, 460 (1984). See also J. R. Ackerhalt, Y. B. Band, J. S. Krasinski, and D. F. Heller, Opt. Lett 13, 646 (1988) which discusses aspects of this invention.
Notwithstanding the high cost and complicated construction of the prior art devices for obtaining Raman output, it is still desirable to obtain such Raman output, for several reasons. Intracavity Raman conversion is an effective means of (1) producing first (or second) Stokes shifted radiation with high quantum conversion of frequency (2) extracting energy efficiently from low emission cross-section media: (3) effecting significant beam cleanup in the sense that the Raman beam is comprised of fewer spacial modes than the fundamental pump beam: and (4) producing short Raman shifted pulses having adjustable time durations.
An additional advantage results when an intracavity Raman laser is used as a non-linear output coupler for low cross-section gain media In this case, the flux in the pump laser cavity can build up to a set maximum level before outcoupling turns on. The Raman laser thus acts as a power limiter for the pump laser. This permits extraction of the Raman output to take place under more efficient high fluence conditions without risk of optical damage.